The present invention relates to the preparation of single-crystal diamond films and more particularly to such films of isotopically pure diamond.
High thermal conductivity diamond, such as type IIA natural diamond, is characterized by a very high degree of purity and is reported to have a thermal conductivity at 25.degree. C. (298.degree. K.) on the order of about 21 watts/cm.sup..degree. K. Such high thermal conductivity diamond is useful as a heat sink material, such as in the backing of semi-conductors.
Despite its high costs, natural type IIA diamond has been employed as a heat sink material because it has the highest known thermal conductivity. Conventionally-produced high pressure/high temperature synthetic gem diamond can be produced with a similarly high thermal conductivity. For the most part, diamond prepared by low-pressure chemical vapor deposition (CVD) processes are not single crystal diamond and have materially lower thermal conductivities, typically in the order of 12 watts/cm.sup..degree. K. at about 300.degree. K. (hereinafter sometimes referred to as "room temperature conductivity"). Recently, however, the growth of single crystal epitaxial polycrystalline CVD diamond for semi-conductors has been developed as reported in commonly-owned application Ser. No. 479,486, filed Feb. 13, 1990, the disclosure of which is expressly incorporated herein by reference.
U.S. Pat. No. 3,895,313 discloses various diamond materials which allegedly have high thermal conductivities and which allegedly are useful as optical elements for very high-power laser beams. In particular, this citation states that synthetic diamonds grown from isotopically pure carbon-12 or carbon-13 would be useful in this way, with room temperature conductivity values in the range of 10-20 watts/cm.sup..degree. K. being mentioned. However, no methods for the preparation of such diamond are suggested.
Commonly-assigned application Ser. No. 448,469 discloses a method for preparing single crystal diamond of very high chemical and isotopic purity by depositing a layer of diamond on a substrate by chemical vapor deposition methodology using a feed mixture of hydrogen and a hydrocarbon containing isotopically pure carbon-12 or carbon-13. The thus-deposited diamond layer is removed from the substrate. The diamond is converted to single-crystal diamond by diffusion under high pressure through a metallic catalyst/solvent to a region containing a diamond seed crystal. This citation reports the production of single crystal 0.95 carat diamond which analyzed to 99.93% carbon being the C-12 isotope when made from methane having an isotope distribution of 99.96% C-12 and 0.04% C-13. The room temperature conductivity of this single crystal product is reported at 31.5 watt/cm.sup..degree. K.